For two years American police departments have endured relentless attacks from the Obama administration, its media allies and the Black Lives Matter movement alleging that U.S. law enforcement is a racist, deadly threat to African-Americans. A handful of disturbing videos depicting police shootings helped galvanize widespread hostility to law-enforcement officers, and cops began backing away from the proactive policing that stops crime but has been repeatedly denounced as racial oppression.The result, especially in the first half of this year, has been an appalling increase in shootings and murders in many cities across America. Most of the victims, in this poisonous era spawned by Black Lives Matter, have been black. Now the consequences of this stream of falsehoods about police may be spinning out of control, with the assassination of five police officers in Dallas last week and the attacks on cops in other cities since then.

Make no mistake: Assertions about systemic, deadly police racism are false. That has been true throughout the period following the shooting of Michael Brown in Ferguson, Mo., in 2014; recall that the cop involved was ultimately exonerated by the Justice Department. But no number of studies debunking this fiction has penetrated the conventional story line.